Seismic activity at abandoned mines

On November 2005 SYNAPSE issued a preliminary study (scientific report) to Soam Consultant Co., Ltd., Korea, reflecting the results of 6 weeks monitoring of seismicity on the abandoned mines of Kumho area.

This report contains the results of location of seismic events registered by the array of REF TEK 130 digital recorders equipped with accelerometers and seismometers. There were 9 3-component sensors altogether, placed in 6 points of registration in the area of former Kumho Mine which has been put out of operation since 2001. The objective of observation is detection of the probable induced deep seismicity related to the abandoned duffers.

All plots in this report were made using graphics subsystems of SNDP software (main 1D graphics tool, 2D plot tool, 3D Surfer subsystem and Cluster subsystem).

Seismic signals resulting from human activity were recorded in the seismic array area during the daytime. Some signals were easily identified. Spectrums of these signals have clear maximum peaks at frequencies of 25-30 Hz, 50-60 Hz and 80-100 Hz. The signal sources are rather strong, traveling of signals over the array is easily traced. Such signals appear to be generated by some powerful electric device. Another type of signals is characterized by a comparatively wide spectrum and lack of clear hodographs. Evidently, the signal sources are weak, multiplex and diffusely distributed. Possibly, the signal results from the thunderstorm activity. Anthropogenic activity produced considerable noise hampering detection of the wanted signal in the daytime recordings. Considerable increase of noise originated presumably by electrical device sometimes lasted for several hours.

While processing data for wanted signal, we searched for signals which could be related to induced seismicity. In the mine working area these seismic signals can be generated by rock bursts and displacements along faults and fractures, which could have been inactive before the start of shaft mining.

For rock bursts as signal sources, the source mechanism is related to fracture. The signal is broadband (10-5000 Hz and even higher), events usually happen at depth greater than critical, ranging from 300 to 600 meters in different regions. The faulting mechanism is related to fracture edges friction, and the signal frequency in this case is lower (the lower bound can be 0.5 – 1 Hz) than in rock burst case, sources can be both deep-seated and near-surface. The expected signal duration in both cases is within the limits of 0.5 sec.

During preprocessing, short (no more than 0.5 sec.) non-periodic signals were picked out.

Intensive human activity in the registration area produce intense background noise, mainly traveling with the sound velocity, and propagating through air. Nevertheless deep clouds of seismic events were detected at a depths of up to 1000 meters. Hundreds of surface events (generated by human activity) were detected as well.

Hypocenters of events were determined within SNDP software using one of the seismic location programs based on Heiger least square fitting. Seismic phases onset times, sensors coordinate and velocity model of environment are used as input data.

Seismic velocity model was based on the results published in [Nguyen Bao.V. Surface-wave focal mechanism, velocity and models of the 29 May 2004 South Korea Earthquake. S53B-0195. Abstracts Received for the AGU 2004 Western Pacific Geophysics Meeting].